One known spot-joining method is friction stir spot welding. In friction stir spot welding a cylindrical punch with a shouldered probe at its tip is rotated and driven into the layers to be joined. Sliding friction between the probe and the workpiece layers causes the layers to soften and plasticise without melting, and the rotation of the probe displaces the material and causes the plasticised portions of the two layers to intermingle. When the punch is withdrawn and the workpiece allowed to cool, the intermingled plasticised portions harden and produce a welded joint between the two layers.
Recently, the softening of material through a friction stir mechanism has been used in relation to blind riveting. Rather than pre-drilling a hole for the rivet, the rivet is rotated and driven into the workpiece so as to cause friction stir softening of the material. The rivet is then driven through the workpiece and, once the material has cooled, the rivet is set in the usual manner. In addition, the possibility of driving a fastener (either for fastening two or more layers of a workpiece together or for providing an anchoring point such as a threaded stud in a workpiece of one or more layers) and joining it to the workpiece by friction stir welding has also been explored. In this case, the fastener is made of a material of similar hardness to one of the layers of the workpiece so that the fastener itself is plasticised and intermingles with the plasticised portion(s) of the workpiece.
Present spot-joining machines and methods which utilise friction stir softening, such as those above, utilise at least one actuator (a motor) to produce the required rotation of the fastener or welding punch and at least one additional actuator to produce the linear motion required to drive the fastener or tip of the punch into the workpiece. Use of two actuators not only increases the cost of the apparatus, but also significantly increases its bulk and weight. The added weight may limit the performance or life of robotic arms on which the apparatus is mounted, and the added bulk may prevent the apparatus from reaching into small spaces. Use of two actuators may also adversely affect the service life and/or maintenance cost of the apparatus.
It is an object of the present invention to obviate or mitigate at least one of the aforesaid disadvantages, and/or to provide an improved or alternative spot-joining apparatus or an improved or alternative method of driving a fastener or performing a friction stir spot welding or clinching operation.